Soil mapping

A soil map is a representation of soil types, for a geographic area,  and/or of their properties (e.g. soil layers (horizons), soil reaction (pH), texture, organic matter content, and soil colour). Traditionally, it is produced on the basis of field observations in a soil survey, at a pre-determined scale (e.g. detailed, reconnaissance or exploratory surveys). The resulting maps are published with a soil report describing the status of the soils and their distribution over the geographic area, as generalised at the scale of the map. The map legend commonly groups and describes the main soil types, of which the distribution, properties and capabilities are explained in detail in a soil survey report. Typically, results of "traditional" soil surveys are presented as polygon maps. These maps are used, amongst other, for assessment of agricultural potential (land evaluation), agricultural extension, and planning the use and management of land (land use and spatial planning).

Nowadays, spatial statistics are commonly used to predict soil properties at various resolutions (e.g. 100m, 250m or 1km), as determined by user-needs. Such "digital soil mapping" work commonly draws on large soil profile databases and a range of environmental co-variates derived from remote-sensing (e.g. elevation, slope, land cover) and auxilliary observations (e.g. climate). Results are presented as grid (raster) maps, for pre-defined soil depths. Digital soil maps commonly show higher spatial detail than traditional soil maps do at similar resolution (or scale); the boundaries between classes are not sharp (discrete) but gradual (continuous). Soil maps produced using (geo)statistical techniques include an estimate of the model uncertainty. Digital soil maps can be used to underpin a wide of applications (e.g. food security and soil carbon dynamics).